Bird watching: for some species, migration is the ultimate marathon

Running a marathon is a great test of human endurance. World-class marathoners increase their metabolic rate five times more than their resting metabolism as they complete the 26.2 miles in about 2 hours.

In what appears to be a never-ending effort to test the limits of endurance, even tougher competitive events have been created. Go on a 2.4 mile swim and 112 mile bike ride and you have the Ironman Triathlon. Other athletes run ultramarathons, long distance mountain races and races to see how far you can run in 24 hours.

For all of these events, the metabolic rate reaches five times the resting rate, so energy reserves are depleted. These athletes are helped along the way with food and water.

As astonishing as these sporting exploits are, they pale in comparison to bird migrations. A migrating bird burns energy faster than a marathon runner. Flight is extremely difficult, and a bird in flight increases its metabolism by at least eight times its rate of rest.

Some birds such as bar-tailed godwits, semipalmated sandpipers, and warblers migrate over long stretches of ocean, flying non-stop for three or more days. These birds must carry all the food and water they need. There are no run volunteers handing out cups of water or chunks of food. If these migrating birds touch the water, they die. Their energy reserves are depleted.

Let’s put ourselves in perspective by considering the non-stop flight of a semipalmated sandpiper from Cobscook Bay to the mouth of the Amazon River in Suriname. This distance is about 2,500 miles – almost 100 marathons – in three or four days. Even this feat has nothing to do with the bar-tailed barges that fly 7,200 miles non-stop from Alaska to New Zealand.

Birds that migrate overland may stop and replenish their fat stores, but they still do the equivalent of five or 10 marathons in a night. Not too bad!

How can birds make these extraordinary flights?

First, birds have much more efficient respiratory systems than mammals, reptiles, and amphibians. The flow of air through our lungs is a two-way flow. The problem is, we are never able to completely evacuate the air from our lungs that has been depleted of oxygen when we breathe out. When we inhale a new breath with a lot of oxygen, it mixes with the residual oxygen-poor air that we couldn’t breathe out.

Birds are unique in that they have a one-way flow of air in their lungs. Through two sets of air sacs, a packet of air passes first into the posterior air sacs, then through the lung, then into the anterior air sacs, and finally out of the trachea.

To feed a migrating leg, birds stuff themselves with fat. The breakdown of fat releases energy as well as water. It is not uncommon for some birds to double their lean weight by greedily storing fat. Fat is stored mainly under the skin of the chest and stomach. Some birders can assess a bird’s fat load by silhouette. Really big birds wriggle when they walk.

We learn more about the changes in the body’s organs during migration. Before migration, some birds increase the length of their intestines. This change allows them to gain weight faster.

After growing properly, some shorebirds shrink their intestines and gizzards. Since birds do not feed on a long flight, the narrowing of the intestine reduces weight.

At the same time, the muscles of the flight on the chest and the heart increase in size. Some birds also increase the size of their lungs.

Once the birds arrive at their destination, they restore their organs to their original size.

How do birds that fly nonstop for days cope with sleep deprivation? Birds can shut off one side of the brain for a few seconds at a time. The brain switches sides, essentially providing hundreds of mini-naps as the birds migrate.

Herb Wilson has taught ornithology and other biology courses at Colby College. It welcomes comments and questions from readers on [email protected]


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